This application claims priority under the Chinese Application No. 00105932.7 that was filed on Apr. 21, 2000.
This present invention belongs to parallel machine tools in which kinematic legs are arranged between a base and a mobile platform in a parallel way, and specially related to a parallel structure of a spatial 3-axis machine tool with three Degrees-of-Freedom (3-DOF).
Conventional machine tools are characterized by a serial arrangement of the feed axes. The individual axes are built one on top of the other, which the lower axis carrying the one above it. Most machine structures are based on this principle. The movement mode of the machine tools is that the cutter and workpiece move along fixed guideways. However, this mode has inherent limitations. For example, in order to maintain high stiffness and therefore high machining precision, traditional machine tools need a bulky machine body, firm columns and steadfast guideway systems, which increase weight, manufacturing cost and transporting difficulty. Moreover, the fact that the cutter can only feed along the guideway limits its machining dexterity.
Recently, Giddings and Lewis (1994) introduced a machine tool called the xe2x80x9cVARIAX Machining Centerxe2x80x9d utilizing the Stewart platform construction with six legs connecting a mobile platform to a base. The Stewart platform has been studied extensively for use as a flight simulator and as a parallel manipulator (D. Stewart, A Platform with Six Degrees of Freedom, Proc. Institute of Mechanical Engr., London, England, Vol. 180, pp. 371-386, 1965). Other variations of the Stewart platform have also been proposed. Behi (F. Behi, Kinematics Analysis for a Six-Degree-of-Freedom 3-PRPS Parallel Manipulator, IEEE J. of Robotics and Automation, Vol. 4, No. 5, pp. 561-565, 1988.) described a 6-DOF configuration with three legs where each leg consists of a PRPS chain. Hudgens and Tesar (J. Hudgens and D. Tesar, A Fully-Parallel Six Degree-of-Freedom Micromanipulator: Kinematics Analysis and Dynamic Model, Proc. 20xe2x80x2 Biennial ASME Mechanisms Conf., Vol. 15-3, pp. 29-38, 1988.) investigated a device with six inextensible legs where each leg is driven by a four-bar mechanism mounted on the base. Pierrot, et al. studied a parallel manipulator using spatial parallelograms (F. Pierrot, Reynaud, and A. Fournier, xe2x80x9cDELTA: A Simple and Efficient Parallel Robot,xe2x80x9d Robotica, Vol. 8, pp. 105-109, 1990). Most of these six-DOP parallel manipulators with the Stewart platform consist of six legs connecting a mobile platform to a fixed base by spherical joints. These six-legged manipulators have the following disadvantages:
1. their direct kinematic problems are difficult to solve;
2. position and orientation of their mobile platform are coupled;
3. their workspace is relatively small;
4. they have limited dexterity and small tilting angles; and
5. there is an inherent danger of strut collision.
Many parallel machine tools utilize the Stewart platform construction. For example, in EMO ""97 more than ten parallel machine tools are disclosed which have been developed by universities and companies. These tools have the above disadvantages. Most are based on the 6-DOF Stewart platform to achieve five-axis motion. This increases problems associated with kinematics, dynamics, calibration and cost. Recently, parallel machine tools having less than 6 DOF (e.g. 3-DOF) have been developed by many universities and companies. For example, a three axis parallel machine with three translational DOF is disclosed by Mori E. and Iwabuchi H., (Prallel Mechanism Drilling Machine (three degrees of freedom), ditto, 1997). Additionally, three axis parallel machines with 3-DOF have been developed by the University of Hannover, ETH, ISW, and Hitachi Seiki. However, these 3-DOF parallel machine tools only achieve three translational DOF, and lack flexibility in orientation making the machining process more difficult.
It is an object of the invention to provide a new and improved parallel structure of a spatial 3-DOF machine tool with two translational DOF and one rotational DOF, whose tilting angle can reach xc2x145xc2x0, and can sufficiently meet the user""s demand for three- to five-axis machine tools.
A further object of the invention is to provide a new and improved parallel structure of a spatial 3-DOF machine tool with parallel strut construction for greater rigidity and stiffness.
Another object of the invention is to provide a new and improved parallel structure of a spatial 3-DOF machine tool having low mass thereby enabling high dynamics.
Yet another object of the invention is to provide a new and improved parallel structure of a spatial 3-DOF machine tool having a favorable ratio of mass to be moved versus carrying capability.
A further object of the invention is to provide a new and improved parallel structure of a spatial 3-DOF machine tool having a favorable ratio of dexterity versus tilting angle.
Another object of the invention is to provide a new and improved parallel structure of a spatial 3-DOF machine tool having a vertical or horizontal machine tool frame.
The invention discloses a 3-axis parallel machine tool with two translational DOF and one rotational DOF. Disclosed is a base, a mobile platform to which a tool may be attached, and three kinematic legs. The first and second kinematic legs are in the same plane. Each of the first and second legs has one 3-DOF spherical joint and two 1-DOF joints. The third kinematic leg has one 1-DOF joint and two universal joints. The mobile platform and tool have two translational DOF in the plane defined by the first and second legs, and a rotational DOF with respect to the axis defined by the two joints which connect the first and second legs to the mobile platform.
In one embodiment of the invention, the three legs preferably are constant length struts. Each of the 1-DOF joints consists of a guideway and a slider. The guideway is mounted on the base. The slider can slide along the guideway. The struts comprising the first and second legs are connected to the sliders through revolute joints, and to the mobile platform through spherical joints. The third strut comprising the third leg is connected to the slider and to the mobile platform through universal joints.
In another embodiment of the invention, the three legs in the machine tool preferably are length-variable struts. The struts comprising two of the three legs are connected to the base through revolute joints, and to the mobile platform through spherical joints. The third strut comprising the third leg is connected to the base and to the mobile platform through universal joints.
In yet another embodiment of the invention, the first and second legs preferably are constant length struts. Each of the 1-DOF joints consists of a guideway and a slider. The guideway is mounted on the base. The slider can slide along the guideway. The struts comprising the first and second legs are connected to the sliders through revolute joints, and to the mobile platform through spherical joints. The third leg preferably is a length variable strut. The third leg is connected to the base and to the mobile platform through universal joints.
In another embodiment of the invention, the first and second legs in the machine tool preferably are length variable struts. The struts of the two legs are connected to the base through revolute joints, and to the mobile platform through spherical joints. The third leg preferably is a constant length strut. The 1-DOF joint of the third leg consists of a guideway and a slider. The guideway is mounted on the base. The slider can slide along the guideway. The strut is connected to the slider and to the mobile platform through universal joints.
These and other objects, advantages, and features of the invention will become more apparent from the following description taken in conjunction with the accompanying drawings.